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Interfaces: The SCSI Interface

SCSI interfaces provide for faster data transmission rates (up to 40 megabytes per second through Ultra-Wide, 80MB/sec with LVD Ultra2, and 160MB/sec using Ultra160 etc.) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port - between 7 and 15- so that SCSI is really an I/O bus rather than simply an interface. SCSI is also a multi-threaded interface which offers performance and I/O advantages when building a RAID system.

There are however various aspects to the SCSI specification. The standards are split between two main standards: electrical interfaces (the cabling, connectors etc.) and the command sets (the actual 'language' or protocol the device uses). Together these two facets combine to produce the SCSI definition. Using various combinations of these two main components of the SCSI standard produces the numerous variations of SCSI.

The main SCSI standards used in general are as follows:

 

SCSI-1 :

Also called Standard SCSI - (Narrow Bus Only)

SCSI-2 :

Also called Fast-SCSI - (Narrow Bus Only)

Wide-SCSI-2 :

Also called Fast-Wide SCSI - (Wide Bus)

SCSI-3 :

Also called Ultra-SCSI - (Narrow Bus Only)

Wide-SCSI-3 :

Also called Ultra-Wide SCSI - (Wide Bus)

HVD SCSI :

Also called Differential SCSI

LVD-SCSI :

Also called Ultra2 SCSI
Ultra160
 A specification that doubles the Ultra2 bandwidth from 80MB/sec to 160MB/sec

Ultra320

A specification that doubles the Ultra160 bandwidth

A further standard that incorporates the SCSI standard is Fibre-Channel, particularly FC-AL, which is viewed as a subset of the SCSI protocol as it supports the SCSI-3 protocol command set for use in a mixed storage environment. Better and more detailed technical information on command set structures and electrical interface standards is available from the T10 committee who are responsible for devising and ratifying SCSI standards. The SCSI Trade Association, the Fibre-Channel Loop Community, and the T11 committee (same as T10 but for Fibre-Channel) also have specific technical information available for a greater understanding of the SCSI platform.


Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. A simple example is the several types of connectors supported across the SCSI standards. The apparent complexity of SCSI interfaces and compatibilities however, are follow extremely simple rules that are easily understood. Behind SCSI technology lies a fairly simple conceptual model and once the more 'meaningful' advertising and marketing terms are discarded there rigid specifications. SCSI based sub-systems offer an additional advantage in that they can be easier to use and configure on most platforms than other high-bandwidth I/O busses.

Saturation of the bus that connects the RAID array members by simultaneously writing to, or reading to, every device on the bus is vital to offer the higher performance potential that exists over single drive storage systems. Unlike IDE, SCSI has the ability to accept multiple and simultaneous I/O requests from the host to communicate with multiple targets. This multiple I/O ability allows a RAID controller to write to or read from, every drive on any SCSI bus at the same time. The data from each drive is combined and its RAID parity format is reconstructed into its original format and transmitted to the host. This turns data read from the multiple drives into a single answering data stream in response to a single I/O request from the user or host system. The larger the number of drives, the smaller each fragment, and therefore the smaller the amount of time required to access, read or write to the disks.

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